The present invention relates to an apparatus for separating gas fractions from a gas mixture having multiple gas fractions. In particular, the present invention relates to a rotary valve gas separation system having a plurality of rotating adsorbent beds disposed therein for implementing a pressure swing adsorption process for separating out the gas fractions.
Pressure swing adsorption (PSA) and vacuum pressure swing adsorption (vacuum-PSA) separate gas fractions from a gas mixture by coordinating pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure of the gas mixture in the adsorbent bed is elevated while the gas mixture is flowing through the adsorbent bed from a first end to a second end thereof, and is reduced while the gas mixture is flowing through the adsorbent from the second end back to the first end. As the PSA cycle is repeated, the less readily adsorbed component is concentrated adjacent the second end of the adsorbent bed, while the more readily adsorbed component is concentrated adjacent the first end of the adsorbent bed. As a result, a xe2x80x9clightxe2x80x9d product (a gas fraction depleted in the more readily adsorbed component and enriched in the less readily adsorbed component) is delivered from the second end of the bed, and a xe2x80x9cheavyxe2x80x9d product (a gas fraction enriched in the more strongly adsorbed component) is exhausted from the first end of the bed.
The conventional system for implementing pressure swing adsorption or vacuum pressure swing adsorption uses two or more stationary adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks. However, this system is often difficult and expensive to implement due to the complexity of the valving required.
Furthermore, the conventional PSA system makes inefficient use of applied energy, because feed gas pressurization is provided by a compressor whose delivery pressure is the highest pressure of the cycle. In PSA, energy expended in compressing the feed gas used for pressurization is then dissipated in throttling over valves over the instantaneous pressure difference between the adsorber and the high pressure supply. Similarly, in vacuum-PSA, where the lower pressure of the cycle is established by a vacuum pump exhausting gas at that pressure, energy is dissipated in throttling over valves during countercurrent blowdown of adsorbers whose pressure is being reduced. A further energy dissipation in both systems occurs in throttling of light reflux gas used for purge, equalization, cocurrent blowdown and product pressurization or backfill steps.
Numerous attempts have been made at overcoming the deficiencies associated with the conventional PSA system. For example, Siggelin (U.S. Pat. No. 3,176,446), Mattia (U.S. Pat. No. 4,452,612), Davidson and Lywood (U.S. Pat. No. 4,758,253), Boudet et al (U.S. Pat. No. 5,133,784), Petit et al (U.S. Pat. No. 5,441,559) and Schartz (PCT publication WO 94/04249) disclose PSA devices using rotary distributor valves having rotors fitted with multiple angularly separated adsorbent beds. Ports communicating with the rotor-mounted adsorbent beds sweep past fixed ports for feed admission, product delivery and pressure equalization. However, these prior art rotary distributor valves are impracticable for large PSA units, owing to the weight of the rotating assembly. Furthermore, since the valve faces are remote from the ends of the adsorbent beds, these rotary distributor valves have considerable dead volume for flow distribution and collection. As a result, the prior art rotary distributor valves have poor flow distribution, particularly at high cycle frequencies.
Hay (U.S. Pat. No. 5,246,676) and Engler (U.S. Pat. No. 5,393,326) provide examples of vacuum pressure swing adsorption systems which reduce throttling losses in an attempt to improve the efficiency of the gas separation process system. The systems taught by Hay and Engler use a plurality of vacuum pumps to pump down the pressure of each adsorbent bed sequentially in turn, with the pumps operating at successively lower pressures, so that each vacuum pump reduces the pressure in each bed a predetermined amount. However, with these systems, the vacuum pumps are subjected to large pressure variations, stressing the compression machinery and causing large fluctuations in overall power demand. Because centrifugal or axial compression machinery cannot operate under such unsteady conditions, rotary lobe machines are typically used in such systems. However, such machines have lower efficiency than modem centrifugal compressors/vacuum pumps working under steady conditions.
Accordingly, there remains a need for a PSA system which is suitable for high volume and high frequency production, while reducing the losses associated with the prior art devices.
According to the invention, there is provided a PSA gas separation system which addresses the deficiencies of the prior art PSA systems.
The gas separation system, in accordance with the invention, comprises a stator and a rotor rotatably coupled to the stator. The stator includes a first stator valve face, a second stator valve face, a number of first function compartments opening into the first stator valve face, and a number of second function compartments opening into the second stator valve face. The rotor includes a first rotor valve surface in communication with the first stator valve face, a second rotor valve face in communication with the second stator valve face, and a number of flow paths for receiving adsorbent material therein which preferentially adsorbs a first gas component of a feed gas mixture in response to increasing pressure in relation to a second gas component of the feed gas mixture. The rotor also includes a number of apertures provided in the rotor valve faces in communication with the function compartments and the ends of the flow paths.
Compression machinery, which can deliver and receive gas flow at a number of discrete pressure levels, is coupled to the function compartments so as to maintain uniformity of gas flow through the function compartments. As a result, mechanical stresses on the compression machinery is reduced, allowing use of centrifugal or axial compression machinery.
The gas separation system includes a number of variable-gap clearance-type valve seals interposed between the first rotor valve face and the first stator valve face and between the second rotor valve face and the second stator valve face. Each variable-gap clearance seal includes a sealing face disposed adjacent a respective one of the rotor valve faces and is pivotal relative to the respective rotor valve face for varying the gas flow rate in accordance with the clearance distance between the sealing face and the respective rotor valve face. Each variable-gap clearance scale also includes an opposing face disposed adjacent the respective stator valve face, with the opposing face and the respective stator valve face together defining a passage therebetween which communicates with one of the function compartments for varying the clearance distance in response to a pressure differential between the passage and an adjacent opposite end. In this way, the seal maintains a smooth pressure transition profile as the flow paths are switched between the function compartments. As a result, equilibrium is maintained between the adsorbent material and the mass transfer front of the gas, and the efficiency of the gas separation process is enhanced.
The gas separation system also includes a number of fixed-gap clearance-type valve seals interposed between the first rotor valve face and the first stator valve face and between the second rotor valve face and the second stator valve face for sealing respective ends of the flow paths. Each fixed-gap clearance seal is substantially identical to the variable-gap clearance seal, including a sealing face disposed adjacent a respective one of the rotor valve faces, an opposing face disposed adjacent the respective stator valve face, and a passage between the opposing face and the stator valve face for pressurizing the sealing face against the rotor valve face. However, the compartment does not communicate with any function compartment, and the fixed-gap clearance seal is fixed at at least one end thereof relative to the respective rotor valve face so as to restrict variations in the clearance gap and to prevent gas leakage from each flow path end passing the sealing face.
In one embodiment of the invention, each variable-gap clearance-type valve seal is positioned between adjacent blowdown function compartments and consists of an elongate slipper having a sealing face and an opposing face extending between the ends of the slipper. Each slipper is pivotally coupled adjacent one of the respective slipper ends to the respective rotor valve face, and includes a resilient biasing element positioned equidistantly between the slipper ends and extending between the stator valve face and the respective opposing slipper face. Further, each passage comprises a compartment defined by the respective stator valve face, the opposing faces of adjacent sealing elements, and adjacent biasing elements, and provides a linear pressure transition profile, at the flow path ends, between the pressure of one of the adjacent blowdown compartments and the pressure of the other of the adjacent blowdown compartments. Since each flow path end opens fully to one of the adjacent blowdown compartments prior to traversing the sealing face of the valve seal, the pressure at the end of each flow path drops linearly from the pressure it attained prior to traversing the sealing face to the pressure of the other of the adjacent blowdown compartments.
In another embodiment of the invention, each variable-gap clearance-type valve seal is positioned between adjacent pressurization function compartments, includes a resilient biasing element positioned at each slipper end and extending between the stator valve face and the respective opposing slipper face. Each passage comprises a compartment defined by the respective stator valve face, the opposing faces of adjacent sealing elements, and the respective biasing elements, and includes an aperture positioned equidistantly between the slipper ends and extending through the slipper between the respective sealing face and the respective opposing face so as to provide a linear pressure transition profile, at the flow path ends, between the pressure of one of the adjacent pressurization compartments and the pressure of the other of the adjacent pressurization compartments. Since each flow path end opens fully to one of the adjacent pressurization compartments prior to traversing the sealing face of the valve seal, the pressure at the end of each flow path increases linearly from the pressure it attained prior to traversing the sealing face to the pressure of the other of the adjacent pressurization compartments.
In operation, a feed gas mixture, including a first gas component and a second gas component, is delivered to the rotor flow paths through the first rotor-stator valve surface pair, and the rotor is rotated at a frequency so as to expose the gas mixture in each rotor flow path to cyclical changes in pressure and direction of flow. These cyclical changes cause the more readily adsorbed component of the feed gas to be exhausted as heavy product gas from the first rotor-stator valve surface pair and the less readily adsorbed component to be delivered as light product gas from the second rotor-stator valve surface pair. To enhance gas separation, light reflux exit gas is withdrawn from the second rotor-stator valve surface pair and is returned after pressure letdown to the second rotor-stator valve surface pair.
In order for the flowing gas streams entering or exiting the function compartments to be substantially uniform in pressure and velocity, the feed gas is delivered to the rotor flow paths, through the clearance seals, at plurality of incremental feed gas pressure levels. Similarly, the heavy product gas is exhausted from the rotor flow paths as countercurrent blowdown gas, through the clearance seals, at a plurality of decremental exhaust gas pressure levels. Preferably, the light reflux exit gas is withdrawn from the rotor flow paths, through the clearance seals, at a plurality of decremental light reflux exit pressure levels and is returned to the rotor flow paths as light reflux return gas, through the clearance seals, at pressure levels less than the respective light reflux exit pressure level.
Preferably the rotor also has a large number of adsorbers such that several adsorbers are exposed to each pressure level at any given moment. During pressurization and blowdown steps, the pressures of the adsorbers passing through each of these steps converge to the nominal pressure level of each step by a throttling pressure equalization, through the clearance seals, from the pressure level of the previous step experienced by the adsorbers. Flow is provided to the adsorbers in a pressurization step or withdrawn in a blowdown step by the compression machinery at the nominal pressure level of that step. Hence flow and pressure pulsations seen by the compression machinery at each intermediate pressure level are minimal by averaging from the several adsorbers passing through the step, although each adsorber undergoes large cyclic changes of pressure and flow.